Add UT with RSpec ShowCase plugin

[pavel-mikula-sonarsource]

There exists a contract about formatting that is legal to use by rule descriptions, and products are expected to support it.

We need to add UT to validate compliancy with this contract.

Why?

We should make sure we

• comply with this contract,
• are able to automatically catch new features,
• and are aware of any elements that are not supported yet.

It will help reduce friction and bring clarity to situations like this: #5414

What?

The UT should

• Download the latest available version of https://github.com/SonarSource/sonar-rspec-showcase-plugin - the version should not be fixed, otherwise, it spoils the purpose of discovering new topic to cover
• Load all rule specifications
• Try to convert each of them to XAML

If anything fails, there should be a ticket created for the specific feature and some string-replace mechanism to suppress this particular error. The suppression should make sure, that another error in the same file is discovered and handled separately.